Apparatus for measuring surface roughness



25, 1947. MOONEY 2,417,988

APPARATUS FOR MEASURING SURFACE ROUGI'H IESS Filed June 30, 1945 INVENTOR. MEL V/IV MOO/V5 ATTORNEY Patented Mar. 25, 1947 s PATENT OFFICE ArranA'rUs FOR MEASURING SURFACE nouomvass I Melvin Mooney, Lake Hiawatha, N. 1., assignor to I l United States Rubber Company, New York.

N. Y., a corporation of New Jersey Application June so, 1945, swarm. 602,497

4 Claims. (01. 73-37) This invention relates to an apparatus for taining a quantitative measure of the roughness, or rugosity, of a surface. The apparatus herein described will be referred to as a rugosimeter.

In the manufacture of articles from natural or synthetic rubber the roughness of the surface obtained following calendering operations is an important criterion in determining the suitability of a given stock for certain applications. Thus, a calendered stock which would be smooth enough for the manufacture of tires might be much too rough to be used in the manufacture of waterproof footwear. In the past it has'been necessary to rely upon the individual judgment of skilled operators as to the relative roughness of rubber stocks, It is an object of this invention to provide a means of obtaining objective measurements of the roughness, or rugosity, of surfaces such as those of calendered natural or synthetic rubber, which will be free of the uncertainties associated with subjective observations. It is also an object of this invention to provide an apparatus which will have essentially the same sensitivity over the wide range of rugosity values encountered in the manufacture of rubber articles.

The term rugosity as used herein is a technical term for surface roughness. Consider an idealized surface which is sinusoidal in profile. Then the height of the hills above the valleys is called the "rugosity height. For a real surface, generally not sinusoidal, its rugosity height is taken to be the height of a sinusoidal surface which would give the same measurement in this rugosity test. The measurement made is essentially the resistance to flow of air between the rough surface and a smooth surface resting on top of it.

The invention is particularly described in reference to the accompanying drawings, in which:

Fig. 1 is an elevational view of an apparatus embodying the invention, parts of which are shown in cross section;

Fig. 2 is a plan view of the bottom face of acontact platewhich constitutes the smooth surface resting on top of the rough surface to be tested as shown in Fig. 1; and

Fig. 3 is an illustration of another form of the invention.

In the operation of the device shown in Figs. 1 and 2, air or other gas under a small superatmospheric pressure is supplied to the inlet ll) of a continuously adjustable aerodynamic resistance II, the outlet I! of which communicates with a pressure indicating device l3 and with 9.

ob- I contact plate l4 so constructed that the gas in escaping must pass between the smooth surface l5 of the plate and the surface I6 of the test sample, whose rugosity is to be measured. The

phrase superatmospheric pressure as used herein indicates the difference between atmospheric pressure and some pressure higher-than atmospheric and as so used it is equivalent to what is often referred to as positive gauge pressure.

Under the test conditions the following relation will be true:

where Rx--the resistance offered to the flow of gas between the smooth surface of the contact plate and the surface whose rugosity is to be determined;

Rv=the resistance offered by the adjustable aerodynamic resistance to the flow of gas through it;

Po=absolute pressure of gas supplied to the adjustable resistance;

Px=absolute pressure of gas at outlet of adjustable resistance; and

Pa=atmcspheric pressure.

As shown by the above equation the aerodynamic resistance Rx of the passage or passages between the contact plate l4 and the surface it (which is an inverse measure of the rugosity of the surface) is directly proportional to the observed value Rv of the aerodynamic'resistance II when the. ratio between the pressure drop (Pr-Pa) across the test sample and the pressure drop (Po-Pr) across the variable resistance is constant. Accordingly my apparatus is provided with means for establishing any selected ratio between such pressure differences, and means for indicating the relative value of the continuously adjustable aerodynamic resistance corresponding to the selected ratio. Under the test conditions, when the pressures corresponding to the selected ratio are established, the observed value of the resistance II will be a measure of -the rugosity of the surface IS. The indicator, 1. e., the dial or scale, of the variable resistance may be calibrated to indicate standard rugosity values as will hereinafter be explained. I

Again referring to the equation '(1), when P:rPa is made equal to (Po-Pa), then P, P,, P P, becomes equal to unity, and R=Ru, that is, the

acraasa' 3 resistance oiiered to the flow of gas between the contact plate and the surface whose rugosity is being measured is equal to the resistance value of the adjustable resistance. This is the most convenient condition under which to use the apparatus, but other ratios between thepressure drops may be used. I I

Fig. 1 is an elevational view of one form of the apparatus which comprises a means, such as a pressure regulator I9, for supplyin gas at constant super-atmospheric pressure to the adjustable aerodynamic resistance II, a pressure indicating device I I, and a contact plate I4 communicating with each other and with the outlet I2 of the aerodynamic resistance II by means of conduits l1 and I8.

The pressure regulator It comprises a cylindrical container I9 having a float valve for cooperating with its upper edge 2| as a valve seat. The valve 20 is provided with a rod 22 which has a cup 23 attached to its lower end and immersed in a liquid 2| contained in dash pct 25. The valve 20 may be weighted by depositing weights in the cup 23 so that the valve can be made to maintain the pressure within regulator I8 at any desired superatmospheric pressure.

The pressure regulator I6 is connected by conduit 26 to a source of air or other gas at a pressure above that which is to be maintained in the regulator I6. As a. precaution to prevent the intrusion of foreign matter into the control valve which constitutes the aerodynamic resistance II,

the gas supply line may be provided with a trap 21. The air pressure regulator I6 is connected by conduit III to the inlet of the adjustable aerodynamic resistance II.

The adjustable aerodynamic or air fiow resisttance I I comprisesa standard needle valve which is provided with a fixed pointer 28 and a graduated scale 29 attached to and movable with the manually rotatable shaft 30 of the needle valve.

The outlet of the needle valve is connected by tube. I! to the pressure indicating device I3 which is a conventional manometer, one leg of which consists of a reservoir 3| filled with .a manometer liquid 32 which in the present apparatus is a light machine oil, and the other leg of which is an inclined tube 33 provided with a graduated scale 34.

Figure 2 is a bottom face view of contact plate I4 which is a fiat metal cylinder having a, central orifice 38 communication with conduit I8 and a cylindrical central cavity 31 having a radius 11 surrounded by an annular ridge I5 having an inner radius 11 and an outer radius r: and having a smooth planar lower surface. When used in the determination of rugosity the contact plate I4 is placed with its lower surface I5 resting on the surface I6 of the sample to ,be tested so that the air flowing out from cavity 31 must pass be-v tween the plate surface Iiand sample surface I6.

In my apparatus it is essential to provide means for comparing the pressure drop across the aerodynamic resistance II with the pressure drop between the outlet I2 and the atmosphere and means for establishing a selected ratio between these two pressure drops. In the apparatus shown in Fig. 1, the pressure at the inlet to the resistance is maintained constant and the manometer I3 is used to measure the supply pressure Po and the pressure at the outlet I2. Under such conditions any selected readings of the pressures Po and P2011 the manometer provide the essential comparison.

4 The preferred adjustments of the manometer Is and the pressure regulator II are such that the manometer reading corresponding to the pressure Po comes at a predetermined convenient point on the manometer scale. To secure this adjustment, the liquid level in the manometer II is first adjusted to coincide with a point selected as the zero reading on scale 24 when the air pressure is oil and orifice II in contact plate I4 is open to the atmosphere. The air pressure is then turned on, valve II is opened wide, and

opening 88 is closed with a suitable stopper. The

pressure regulator It is then adjusted by adding to or taking weights out of cup 28 until the liquid placing the sample IS on a fiat glass plate, or

other smooth hard surface, and placing the con-' tact plate I4 on the top surface of the sample.

The valve II is then adjusted until the liquid level in the manometer I3 stands at the midpoint between the lower and the upper index mark on the scale 94. Thus the midpoint will be 10 when the lower and upper indices are zeroand 20, respectively. Readings of the needle valve scale 29, for various samples, will indicate the relative rugosity of the several samples. The sample giving a valve scale reading indicating the greatest valve opening will have the greatest rugosity and of course the rugosit of the samples diminishes in proportion as the observed valve openings diminish.

To convert the needle valve positions as indicated by the graduated scale 29 and pointer 28 into corresponding standard rugosity values, it is necessary to calibrate the apparatus against known standards. For that purpose a series of capillary tubes of'known dimensions are substituted one after another for the unknown aerodynamic resistance Rx, by connecting such tube to conduit I8 in place of the contact plate I4.

for the tubes against the calculated rugosity heights. The calibration curve through these points is almost linear, indicating that the sensitivity of the apparatus is essentially constant over a wide range of'rugosity values.

The rugosity value corresponding to the valve setting for which P::-Pa= /2(P0-Pa) when a capillary tube is substituted for a given contact plate may be calculated from the following equation:

h=rugosity value, rz=outer radius of annular contact surface l5 of contact plate I 4, a

r1=inner radius of annular contact surface l5 of contact plate l4,

ru=radius of capillary L=length of capillary.

For instance, when the inner and outer radii of the planar surface of the contact plate H are 4.0 cm. and 9.0 cm., respectively, and five capillary tubes each having a length of cm. and

' having internal radii of .04 cm., .08 cm., .16 cm.,

.24 cm., and .32 cm., respectively, are used for standardization, it is found that these capillaries are equivalent to surfaces having rugosity heights of .00792 cm., 0.0200 cm., .0503 cm., .0864 cm., and .1270 cm. respectively.

Commercial calendered rubber stocks commonly have rugosity heights below .08 cm.; those with rugosity heights above .02 cm. are considered too rough for footwear, while stockshaving rugosity heights above .05 cm. are considered too rough for tire stocks.

It has been proposed heretofore to measure the rugosity of surfaces by supplying air at constant pressure to a fixed aerodynamical resistance represented by a capillary tube and thence to an annular contact plate so constructed that the gas in escaping must pass between its smooth surface and the surface of the sample to be tested. The resistance to the flow of air between the contact plate and the sample is determined by measuring the pressure drop between the source of air at constant pressure and the contact plate. The difficulty with such an apparatus is that its sensitivity varies widely over the range of ru gosity or roughness encountered in rubber practice. Therefore, the accuracy of the rugosity one arm connected to the outlet 44, and the other arm is open to the atmosphere, so that the dif ference in liquid level in the arms indicates the drop in pressure between the outlet and the atmosphere. With the arrangement of the tubes as shown, the drop in pressure across the valve 42 is indicated by the liquid level 45 in .the tube 40, and the pressure in the outlet 44 is indicated by the liquid level 46 in the tube 4|, and the respective pressures can be thus compared either by the relative positions of the two levels or by reference to graduations on the tubes. For convenience the manometers are so constructed and adjusted that the liquid levels 45, 45346 and 46 in the two manometers are at the same level when the pressure differences are zero and the apparatus is open to the atmosphere. This is done by moving the manometer 4|, up or down and clamping it in position with the clamp 41'.

values thus obtainedvaries considerably throughout the range of rugosity encountered in the practice of measuring the roughness of rubber surfaces. I have found however, that by using a continuously variable aerodynamic resistance such as a needle valve in place of the fixed resistance used in the prior art and by adjusting said needle valve so that its resistance to the flow of air or gas therethrough is made directly proportional to the resistance to the flow of air between the contact plate and the sample, accurate rugosity values can be obtained over a wide range.

The second form of the invention, shown in Fig. 3 of the drawings, has a further advantage over rugosimeters heretofore used in that it does not require the maintenance of a precisely uniform or constant supply pressure at the inlet to the aerodynamic resistance, because it provides the necessary comparison between the pressure differences across the two resistances without determination of any manometer scale readings and without reference to any fixed points on the manometer tubes, even though the supply pressure fluctuates somewhat. Accordingly, in this device the pressure regulator in the supply line may be eliminated, provided that a throttling or pressure-reducing valve is put in the supply line so as to provide a fairly steady gas pressure within the range of the manometer used.

The rugosimeter shown in Fig. 3 works on the same principle as that disclosed in Fig. 1. structural difference is thatmanometers and 4| are employed for comparing the pressure drops in place of manometer l3 disclosed in Fig. 1. The manometer 40 is connected across the aerodynamic resistance valve 42 and the difference in levels of the liquid therein indicates the drop in pressure between the inlet 43 and the outlet 44 of the valve. The manometer 4| has The i The flexible tube 48 will permit the movement. As previously stated the most convenient ratio of pressure drops to be used in carrying out the test is unity, a condition obtained when the drop in pressure across the valve 42 equals the drop in pressure from the outlet 44 to the atmosphere. When such condition is obtained the liquid levels 45 and 46 of the respective manometers are at the same level, thus affording a convenient criterion of the condition sought. The relative rugosities of several samples to be tested are indicated by the valve scale 41 when it is so adjusted that the liquid levels 45 and 46 are the same. In the event it is desired to calibrate the valve scale 41 to standard rugosity values it can be done with the use of the capillary tubes in the same manner as described in reference to Fig. 1.

Although the above invention has bee-n described as adapted to the measurement of the roughness of approximately plane surfaces it should be apparent that by suitably modifying to the superficial outline of the surface to be tested.

It should also be apparent that the advantages which accrue from the use in my rugosimeter of a continuously adjustable aerodynamic resistance in place of the fixed resistance used heretofore can be obtained in other modifications which fall within the broad scope of my invention as indicated by the appended claims.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. A rugosimeter comprising a conduit, a contact plate, a continuously adjustable aerodynamic resistance within the conduit, means for supplying gas at superatmospheric pressure to the inlet of the conduit, the outlet of the conduit being connected to the contact plate adapted to be maintained in contact with the surface to be tested in such a way that the gas in escaping from the conduit must pass between the smooth surface of said plate and the surface to be tested, means for indicating the relative resistance value of said adjustable aerodynamic resistance, and means for comparing the gas pressure drop across said aerodynamic resistance with the gas the inlet of said conduit, the outlet of said conduit being connected to the contact plate adapted to be maintained in contact with the surface to be tested in such a way that the gas in escapin from said conduitmust pass between the smooth surface of said plate and the surface to be tested, means for indicating the relative resistance value of said adjustable aerodynamic resistance, and means for indicating a selected ratio between the gas pressure drop across said aerodynamic resistance and the gas pressure drop from the outlet of said conduit to the atmosphere.

3. A rugosimeter comprising a conduit, a contact plate, a continuously adjustable aerodynamic resistance within the conduit, means for supplying gas at a known constant superatmospheric pressure to the inlet of such conduit, means for indicating the superatmospheric pressure at the outlet of said conduit, the outlet of said conduit being connected to the contact plate adapted to be maintained in contact with the surface to be tested in such a way that the gas in escaping from said conduit must pass between the smooth surface of said plate and the surface to be tested, and means for indicating the relative resistance value of said aerodynamic resistance.

4. A rugosimeter comprising a conduit, a con- 8 tact plate, an adiustable valve within said conduit, said valve being adapted to offer a continuously adjustable resistance to the flow of gas through said conduit, said valve "being provided with means for indicating the relative opening of the valve, means-for supplying air at a known constant superatmospheric pressure to the inlet of said conduit, means for indicating the superatmospheric pressure at the outlet of said conduit, the outlet of said conduit being connected to the contact plate adapted to be maintained in contact with the surface to be tested so that the gas in escaping from said conduit must pass between the smooth surface of said plate and the surface to be tested.

' MELVIN MOONEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Lewis May-18,1944 

